This application is for continuation of a project aimed at investigating the mechanisms leading to decreased hepatic growth in rat fetuses with intrauterine growth retardation (IUGR). It focuses on fundamental differences in growth regulation between fetal and adult hepatocytes in primary culture. The proposed studies are based on several key findings which came out of the first cycle of this grant: [1] Cultured fetal rat hepatocytes, in contract to cultured adult rat hepatocytes, exhibit autocrine growth stimulation; [2] Fetal hepatocytes display constitutive expression of the proto-oncogene, c-myc; [3] Insulin and Transforming Growth Factor-alpha (TGF-alpha) potentiate the DNA synthesis of hepatocytes above basal levels in a dose-responsive manner while TGF-beta inhibits fetal hepatocyte growth; [4] Insulin potentiation of DNA synthesis is abolished in hepatocytes from growth retarded fetuses. Based on these findings, we have formulated the following corresponding specific aims: [1] Study the ontogeny of three signal transducing protein kinases (Protein Kinase C[PK-C}, Mitogen Activated Protein Kinases [MAP Kinases] and casein Kinase II [CS-II] which are involved in mitogen activation of growth. Determinations will be made in adult liver, fetal liver (days 17 through 21 of gestation), cultured adult hepatocytes and cultured fetal hepatocytes; [2] Determine the fetal hepatocyte transcription rates for exons I, II and III of c-myc, thereby focusing on the phenomenon of "pausing" which has a critical effect on c-myc transcription. Comparisons will be made with adult rat hepatocytes in primary culture; [3] Determine the effect of insulin, TGF-alpha and TGF- beta on signal transducing kinase activities and c-myc transcriptional regulation (initiation and elongation) in cultures of fetal rat hepatocytes. Comparisons will be made using cultures of adult rat hepatocytes; [4] Do comparative studies using hepatocytes from control IUGR fetuses to determine the response of signal transducing protein kinases and c-myc regulation to insulin as well as to the transforming growth factors., It is anticipated that the results of these studies will shed light on basic mechanisms regulating hepatic growth in the fetus and the nature of perturbations in these mechanisms which accompany IUGR.